Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc., geographical features and their location- changes in critical geographical features (including water-bodies and ice-caps) and in flora and fauna and the effects of such changes

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UTTARAKHAND FLASH FLOOD

• Recently, a glacial break in the Tapovan-Reni area of Chamoli District of Uttarakhandled to massive Flash Flood in Dhauli Ganga and Alaknanda Rivers, damaging houses and the nearby Rishiganga power project.
• In June 2013, flash floods in Uttarakhand wiped out settlements and took lives.

Important  points:

• It occurred in river Rishi Ganga due to the falling of a portion of Nanda Devi glacier in the river which exponentially increased the volume of water.
• Rishiganga meets Dhauli Ganga near Raini. So Dhauli Ganga also got flooded.

Flash Floods:

• These are sudden surges in water levels generally during or following an intense spell of rain.
• These are highly localised events of short duration with a very high peak and usually have less than six hours between the occurrence of the rainfall and peak flood.
• The flood situation worsens in the presence of choked drainage lines or encroachments obstructing the natural flow of water.

Causes:

• It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields.
• Flash Floods can also occur due to Dam or Levee Breaks, and/or Mudslides (Debris Flow).
• In areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat.
• The intensity of the rainfall, the location and distribution of the rainfall, the land use and topography, vegetation types and growth/density, soil type, and soil water- content all determine just how quickly the Flash Flooding may occur, and influence where it may occur.

Glaciers and Floods:

• Retreating glaciers, like several in the Himalayas, usually result in the formation of lakes at their tips, called proglacial lakes, often bound only by sediments and boulders.
• If the boundaries of these lakes are breached, it can lead to large amounts of water rushing down to nearby streams and rivers, gathering momentum on the way by picking up sediments, rocks and other material, and resulting in flooding downstream.

Impact of Climate Change:

• Climate change has driven erratic weather patterns like increased snowfall and rainfall, warmer winters has led to the melting of a lot of snow.
• According to the latest assessment reports of the UN Intergovernmental Panel onClimate Change, glacier retreat and permafrost thaw are projected to decrease the stability of mountain slopes and increase the number and area of glacier lakes.

SOURCE:PIB